Apparatus and method for introducing impurity

ABSTRACT

An impurity introducing apparatus of the present invention includes: a system for introducing an impurity having charges into a target to be processed, the target being a semiconductor substrate or a film formed on the substrate; a system for supplying electrons into the target, the electrons canceling the charges of the impurity; and a system for controlling the maximum energy of the electrons supplied by the electron supply system at a predetermined value or less.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to apparatus and method forintroducing controlled amounts of selected impurity atoms into a surfaceof a target.

[0002] As the number of VLSI devices, integrated with a drasticallyreduced size on a single chip, has been increasing, the thickness of agate insulating film for an MOS transistor has been tremendously reducedthese days. As for a device with a design rule of 0.5 μm, the thicknessof a gate insulating film was once ordinarily set at about 10 nm. In adevice now available with a design rule of 0.25μm, however, thethickness of the film is almost halved, i.e., about 5 nm. A gateinsulating film with such a very small thickness is extremely sensitiveto charge buildup damage caused by implantation of dopant ions into thegate electrode of the device.

[0003] Recently, in order to process a wafer of a much greater sizesatisfactorily and to increase a throughput sufficiently, the maximumdensity of a beam current for an ion implanter has been increased.However, if the density of a beam current, created during ionimplantation, is increased, then a positive charge buildup phenomenon,resulting from an impacting ion beam, gets even more remarkable.Accordingly, to suppress such a phenomenon, the ability of supplyingelectrons should be improved for an electron supply system.

[0004]FIG. 10 illustrates a cross-sectional structure for a conventionalimpurity introducing apparatus. As shown in FIG. 10, a guide tube 12 isprovided to face a wafer 11 held by a wafer holder 10. A tube bias isapplied from a first voltage supply 13 to the guide tube 12. An ion beam14, which has been generated by an ion beam generator (not shown),travels inside the guide tube 12 leftward to impinge on the surface ofthe wafer 11.

[0005] An arc chamber 16 is provided beside the guide tube 12 andincludes a filament 17 therein. A filament voltage is applied from asecond voltage supply 18 to both terminals of the filament 17. An arcvoltage is applied from a third voltage supply 19 to between one of theterminals of the filament 17 and the arc chamber 16. And arc current issupplied from a current source 20 into the arc chamber 16.

[0006] Argon (Ar) gas, for example, is introduced from a gas feed system21 into the arc chamber 16. By supplying the Ar gas or the like into thearc chamber 16 and applying respective predetermined voltages to the arcchamber 16 and to the filament 17, plasma is created inside the arcchamber 16. And electrons, included in the plasma created, are suppliedinto the guide tube 12 to have a certain energy distribution.

[0007] The guide tube 12, first, second and third voltage supplies 13,18, 19, arc chamber 16, filament 17, current source 20 and gas feedsystem 21 constitute an electron supply system 22 for supplyingelectrons to be introduced into the wafer 11. The electrons 23, whichhave been supplied from the arc chamber 16 into the guide tube 12, areattracted to a positive ion beam 14 to be distributed around the beam 14and introduced into the wafer 11 together with the beam 14. The otherelectrons, which have not been attracted to the vicinity of the ion beam14, are also attracted and introduced into the wafer 11 by an electricfield formed between the guide tube 12 and the wafer 11.

[0008] However, the present inventors found that, if ions were implantedinto a gate electrode on the wafer 11 using this conventional impurityintroducing apparatus, the smaller the thickness of a gate oxide film,the higher the percentage of dielectric breakdown caused in the gateoxide film.

[0009] FIGS. 11(a) and 11(b) illustrate relationships between thethickness of a gate oxide film and the percentage of breakdown caused inthe film, in which ions are implanted into the gate electrode of an MOStransistor, exhibiting antenna effect, using the conventional impurityintroducing apparatus. Herein, the “antenna effect” is a phenomenon thatif the area of a gate electrode is set larger than that of a gateelectrode actually formed in a transistor, then a gate insulating filmis affected by the charge of ions and electrons to a higher degree.FIGS. 11(a) and 11(b) illustrate results obtained by implanting the ionsinto p- and n-type semiconductor substrates, respectively. In this case,As⁺ ions are implanted under the conditions that accelerating voltage is20 keV, implant dose is 5×10¹⁵/cm² and a beam current is 10 mA. In theMOS transistor, the area of the gate insulating film is 1×10⁻⁶ mm², thearea of the gate electrode is 1×10⁻¹ mm², and the antenna ratio is1×10⁵.

[0010] In FIGS. 11(a) and 11(b), the abscissas indicate thick-nesses ofthe gate oxide film, while the ordinates indicate percentages ofbreakdown caused in an MOS transistor exhibiting the antenna effect,where the breakdown voltage of the gate oxide film thereof is 8 MV/cm orless. FIGS. 11(a) and 11(b) also illustrate respective results obtainedwith the flux of electrons supplied from an electron supply systemvaried, where the respective fluxes of electrons are represented as 0.5,1.0, 1.5 and 2.0 by normalizing the flux of electrons supplied understandard conditions at 1.0. As can be understood from FIGS. 11(a) and11(b), the smaller the thickness of the gate oxide film, the higher thepercentage of breakdown caused in the gate oxide film, even though theflux of electrons supplied from the electron supply system 22 remainsthe same.

SUMMARY OF THE INVENTION

[0011] A prime object of the present invention is preventing thepercentage of breakdown caused in a gate insulating film from increasingeven if the thickness of the film is reduced.

[0012] To achieve this object, an apparatus for introducing an impurityaccording to the present invention includes: means for introducing animpurity having charges into a target to be processed, which is asemiconductor substrate or a film formed on the substrate; means forsupplying electrons into the target to cancel the charges of theimpurity; and means for controlling the maximum energy of the electronssupplied by the electron supply means at a predetermined value or less.

[0013] The apparatus of the present invention includes the means forcontrolling the maximum energy of the electrons, supplied by theelectron supply means, at a predetermined value or less. Accordingly, itis possible to prevent the target to be processed or the semiconductorsubstrate, on which the target is formed, from being negatively chargedup.

[0014] In one embodiment of the present invention, the impurityintroducing means is preferably means for implanting ions as theimpurity.

[0015] In such an embodiment, it is possible to prevent a negativecharge buildup phenomenon from being caused during the ion implantation.

[0016] In another embodiment of the present invention, if an insulatingfilm with a thickness of t (nm) is formed on the semiconductorsubstrate, then the predetermined value is preferably 2t (eV).

[0017] In such an embodiment, it is possible to prevent dielectricbreakdown from being caused in the insulating film due to the negativecharge buildup phenomenon.

[0018] In still another embodiment, the apparatus preferably furtherincludes means for measuring the energy of the electrons supplied by theelectron supply means.

[0019] In such an embodiment, the energy of the electrons supplied bythe electron supply means can be known.

[0020] In such a case, the energy measuring means preferably includesmeans for measuring the maximum energy of the electrons supplied by theelectron supply means.

[0021] Then, it is easier to control the maximum energy of theelectrons, supplied by the electron supply means, at the predeterminedvalue or less.

[0022] In an alternate embodiment, the energy measuring means preferablymakes the control means control the maximum energy of the electrons,supplied by the electron supply means, at the predetermined value orless based on the measured energy of the electrons.

[0023] In such an embodiment, the maximum energy of the electrons,supplied by the electron supply means, can be automatically controlledat the predetermined value or less.

[0024] A method for introducing an impurity according to the presentinvention includes the steps of: introducing an impurity having chargesinto a target to be processed, which is a semiconductor substrate or afilm formed on the substrate; and supplying electrons into the target tocancel the charges of the impurity. The step of supplying electronsincludes the step of controlling the maximum energy of the electronssupplied at a predetermined value or less.

[0025] In accordance with the method of the present invention, the stepof supplying electrons includes the step of controlling the maximumenergy of the electrons supplied at a predetermined value or less.Accordingly, it is possible to prevent the target to be processed or thesemiconductor substrate, on which the target is formed, from beingnegatively charged up.

[0026] In one embodiment of the present invention, the step ofintroducing an impurity preferably includes the step of implanting ionsas the impurity.

[0027] In such an embodiment, it is possible to prevent a negativecharge buildup phenomenon from being caused during the step ofimplanting ions.

[0028] In another embodiment of the present invention, if an insulatingfilm with a thickness of t (nm) is formed on the semiconductorsubstrate, then the predetermined value is preferably 2t (eV).

[0029] In such an embodiment, it is possible to prevent dielectricbreakdown from being caused in the insulating film due to the negativecharge buildup phenomenon.

[0030] In still another embodiment, the method preferably furtherincludes the step of measuring the energy of the electrons supplied inthe step of supplying electrons.

[0031] In such an embodiment, the energy of the electrons supplied inthe step of supplying electrons can be known.

[0032] In this case, the step of measuring the energy preferablyincludes the step of measuring the maximum energy of the electronssupplied in the step of supplying electrons.

[0033] Then, it is easier to control the maximum energy of theelectrons, supplied in the step of supplying electrons, at thepredetermined value or less.

[0034] In an alternate embodiment, the step of measuring the energypreferably includes the step of controlling the maximum energy of theelectrons, supplied in the step of supplying electrons, at thepredetermined value or less based on the measured energy of theelectrons.

[0035] In such an embodiment, the maximum energy of the electrons,supplied in the step of supplying electrons, can be automaticallycontrolled at the predetermined value or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic representation illustrating the overallarrangement of an impurity introducing apparatus according to anexemplary embodiment of the present invention.

[0037] FIGS. 2(a) and 2(b) are graphs illustrating respectivedistributions of electron energies with arc current and tube biasvoltage varied, respectively.

[0038]FIG. 3 is a schematic representation illustrating how energy andflux of electrons supplied from an electron supply system are measured.

[0039] FIGS. 4(a) and 4(b) are graphs illustrating respectiverelationships between the energy and flux of electrons supplied, whereions are implanted with tube bias voltage and arc current varied,respectively.

[0040] FIGS. 5(a) and 5(b) are graphs illustrating respectiverelationships between an arc current and the percentage of breakdowncaused in a gate oxide film on p- and n-type semiconductor substrates,respectively, to which ions are implanted with the arc current varied.

[0041] FIGS. 6(a) and 6(b) are graphs illustrating respectiverelationships between a tube bias voltage and the percentage ofbreakdown caused in a gate oxide film on p- and n-type semiconductorsubstrates, respectively, to which ions are implanted with the tube biasvoltage varied.

[0042]FIG. 7 is a graph illustrating a relationship between the maximumenergy of electrons supplied by an electron supply system and thepercentage of breakdown caused in a gate oxide film.

[0043]FIG. 8 is a graph illustrating a relationship between thethickness of a gate oxide film and the maximum energy of electrons atwhich breakdown happens in the gate oxide film.

[0044]FIG. 9 is a graph illustrating a relationship between the flux ofelectrons supplied and the percentage of breakdown caused in a gateoxide film.

[0045]FIG. 10 is a schematic representation illustrating the overallarrangement of a conventional impurity introducing apparatus.

[0046] FIGS. 11(a) and 11(b) are graphs illustrating respectiverelationships between the thickness of a gate insulating film and thepercentage of breakdown, where ions are implanted into p- and n-typesemiconductor substrates, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Hereinafter, an impurity introducing apparatus according to anexemplary embodiment of the present invention will be described withreference to FIG. 1. The apparatus of the present invention introducescontrolled amounts of selected impurity atoms into a wafer 101 held on awafer holder 100. According to the present invention, an electron supplysystem of any type may be provided for the apparatus. In the followingembodiment, the apparatus of the present invention is provided with aplasma flood system for supplying electrons, which have been createdfrom plasma, onto an orbit of an ion beam.

[0048] As shown in FIG. 1, a guide tube 102 is provided to face thewafer 101 held on the wafer holder 100. A first voltage supply 103 forsupplying a variable tube bias voltage to the guide tube 102 isconnected to the guide tube 102. The level of the tube bias voltage,supplied from the first voltage supply 103, is controlled by a tube biascontroller 104.

[0049] An arc chamber 105 is provided beside the guide tube 102 andincludes a filament 106 therein. A filament voltage is applied from asecond voltage supply 107 to both terminals of the filament 106. An arcvoltage is applied from a third voltage supply 108 to between one of theterminals of the filament 106 and the arc chamber 105.

[0050] A current source 109 for supplying a variable arc current to thearc chamber 105 is connected to the arc chamber 105. The amount of thearc current, supplied from the current source 109, is controlled by anarc current controller 110.

[0051] An electron parameter sensor 111 for sensing the energy and fluxof electrons supplied and introduced into the wafer 101 is provided forthe wafer holder 100. The electron parameter sensor 111 includes: avariable voltage supply 112; an ammeter 113; and a wafer voltagecontroller 114. The variable voltage supply 112 supplies a voltage tothe wafer holder 100 and the wafer 101 while varying the value thereof.The ammeter 113 measures the amount of a current flowing in the waferholder 100 and the wafer 101. And the wafer voltage controller 114controls the voltage supplied from the variable voltage supply 112 basedon the value of the voltage supplied from the variable voltage supply112 and the amount of current measured by the ammeter 113.

[0052] Argon (Ar) gas, for example, is introduced from a gas feed system116 into the arc chamber 105. By supplying the Ar gas or the like intothe arc chamber 105 and applying respective predetermined voltages tothe arc chamber 105 and to the filament 106, plasma is created insidethe arc chamber 105. And electrons, included in the plasma created, aresupplied into the guide tube 102 to have a certain energy distribution.

[0053] The guide tube 102, first, second and third voltage supplies 103,107, 108, tube bias controller 104, arc chamber 105, filament 106,current source 109, arc current controller 110, electron parametersensor 111 and gas feed system 116 constitute an electron supply system117. The electrons 118, which have been supplied from the arc chamber105 into the guide tube 102, are attracted to a positive ion beam 119and introduced into the wafer 101 together with the beam 119. The otherelectrons, which have not been attracted to the vicinity of the ion beam119, are also attracted and introduced into the wafer 101 by an electricfield formed between the guide tube 102 and the wafer 101.

[0054] As the arc current, flowing between the arc chamber 105 and thefilament 106, increases, the density of the plasma, created inside thearc chamber 105, becomes higher. As a result, the flux of electronssupplied into the guide tube 102 also increases. On the other hand, if ahigher tube bias voltage is applied to the guide tube 102, most of theelectrons 118, supplied into the guide tube 102, come to have higherenergy values.

[0055]FIG. 2(a) illustrates the distribution of electron energy with anarc current varied. In FIG. 2(a), the axis of abscissas indicates theenergy of electrons, while the axis of ordinates indicates the flux ofelectrons supplied. Arrows A, B and C indicate the individual maximumvalues of the electron energy associated with arc currents of 0.5 A, 1 Aand 2 A, respectively. These maximum values can be defined as suchbecause the energy higher than the respective values identified by thearrows A, B and C can be regarded as noise.

[0056]FIG. 2(b) illustrates the distribution of electron energy variablewith a tube bias voltage applied. In FIG. 2(b), the axis of abscissasindicates the energy of electrons, while the axis of ordinates indicatesthe flux of electrons supplied. Arrows A, B and C indicate theindividual maximum values of the electron energy associated with tubebias voltages of 0 V, −5 V and −10 V, respectively. These maximum valuescan be defined as such because the energy higher than the respectivevalues identified by the arrows A, B and C can be regarded as noise.

[0057] As can be understood from FIG. 2(a), the larger the arc current,the larger the total flux of electrons supplied and the higher themaximum energy of electrons. Also, as can be seen from FIG. 2(b), thelower the tube bias voltage (−10 V being herein regarded as lower than 0V), the higher the maximum energy of electrons.

[0058] Accordingly, if an arc current and a tube bias voltage arechanged, then the maximum energy and total flux of electrons introducedinto a wafer can be changed.

[0059] Next, it will be described with reference to FIG. 3 how theenergy and flux of electrons supplied from the electron supply system117 are measured.

[0060] The ammeter 113 measures a total of the flux of electrons,included in the ion beam 119 and implanted into the wafer 101, and theflux of electrons, supplied from the arc chamber 105 into the guide tube118 and then introduced into the wafer 101.

[0061] As the voltage applied from the variable voltage supply 112 tothe wafer holder 100 is increased, the amount of the current measured bythe ammeter 113 decreases, because electrons, having energy lower thanthe applied voltage, can no longer reach the wafer holder 100.

[0062] Accordingly, the amount of current flowing in the ammeter 113 ismeasured with the voltage applied to the wafer holder 100 increased atregular intervals. That is to say, an amount of current, associated witha predetermined voltage applied, is measured. And based on the amount ofcurrent associated with the predetermined voltage applied, a flux ofelectrons associated with a predetermined energy range can becalculated.

[0063]FIG. 4(a) illustrates a relationship between the energy and fluxof electrons supplied, where ions are implanted with a tube bias voltagevaried. In this case, As⁺ ions are implanted under the conditions thataccelerating voltage is 20 keV, beam current is 10 mA and arc current is1 A. FIG. 4(b) illustrates a relationship between the energy and flux ofelectrons supplied, where ions are implanted with an arc current varied.In this case, As⁺ ions are implanted under the conditions thataccelerating voltage is 20 keV, beam current is 10 mA and tube biasvoltage is 0 V.

[0064] As can be understood from FIG. 4(a), the lower the tube biasvoltage, the higher the peak energy (i.e., energy of electronsassociated with the maximum flux of electrons, or the maximum ordinate)and the maximum energy of electrons supplied (i.e., the maximumabscissa). The flux of electrons associated with the peak energy is thelargest when the tube bias voltage is −5 V, but the total flux ofelectrons supplied (i.e., an integrated value of the fluxes ofelectrons) is substantially constant.

[0065] As can be seen from FIG. 4(b), as the arc current increases, boththe maximum energy and the total flux of electrons supplied increase.

[0066] Next, it will be described how the percentage of breakdown causedin a gate oxide film changes if the maximum energy and total flux ofelectrons, introduced into a wafer, are changed by changing an arccurrent and a tube bias voltage. As⁺ ions are herein implanted into agate electrode of an MOS transistor, exhibiting an antenna effect, underthe conditions that accelerating voltage is 20 keV, implant dose is5×10¹⁵/cm² and beam current is 10 mA. In the MOS transistor used for theion implantation, p- and n-type substrates are used, a gate insulatingfilm has an area of 1×10⁻⁶ mm², the gate electrode has an area of 1×10⁻¹mm² and the antenna ratio is 1×10⁵.

[0067]FIG. 5(a) illustrates a relationship between an arc current andthe percentage of breakdown caused in a gate oxide film, which is formedon a p-type semiconductor substrate to have a thickness of 5.0 nm and towhich ions are implanted with the arc current varied. In this case, theions are implanted under the conditions that tube bias voltage is 0 Vand beam current is 10 mA. FIG. 5(b) illustrates a relationship betweenan arc current and the percentage of breakdown caused in a gate oxidefilm, which is formed on an n-type semiconductor substrate to have athickness of 5.0 nm and to which ions are implanted with the arc currentvaried. In this case, the ions are implanted under the conditions thattube bias voltage is 0 V and beam current is 10 mA.

[0068] As can be understood from FIGS. 5(a) and 5(b), the larger the arccurrent, the higher the percentage of breakdown caused in the gate oxidefilm. Thus, considering this and the above characteristics (i.e., thetotal flux and maximum energy of electrons supplied increase with thearc current) in combination, if the total flux or maximum energy ofelectrons supplied increases, then breakdown happens in the gate oxidefilm with a higher percentage.

[0069]FIG. 6(a) illustrates a relationship between a tube bias voltageand the percentage of breakdown caused in a gate oxide film, which isformed on a p-type semiconductor substrate to have a thickness of 5.0 nmand to which ions are implanted with the tube bias voltage varied. Inthis case, the ions are implanted under the conditions that arc and beamcurrents are 1 A and 10 mA, respectively. FIG. 6(b) illustrates arelationship between a tube bias voltage and the percentage of breakdowncaused in a gate oxide film, which is formed on an n-type semiconductorsubstrate to have a thickness of 5.0 nm and to which ions are implantedwith the tube bias voltage varied. In this case, the ions are implantedunder the conditions that arc and beam currents are 1 A and 10 mA,respectively.

[0070] As can be understood from FIG. 6(a) and 6(b), the lower the tubebias voltage (−15 V being herein regarded as lower than 0 V), the higherthe percentage of breakdown caused in the gate oxide film. Thus,considering this and the above characteristics (i.e., the maximum energyof electrons increases as the tube bias voltage decreases) incombination, as the maximum energy of electrons supplied increases, thebreakdown happens in the gate oxide film with a higher percentage.

[0071] Accordingly, as described with reference to FIGS. 5(a), 5(b),6(a) and 6(b), if the total flux or the maximum energy of electronssupplied increases, then the breakdown happens in the gate oxide filmwith a higher percentage.

[0072] Next, a relationship between the maximum energy of electrons andthe percentage of breakdown caused in a gate oxide film will bedescribed.

[0073]FIG. 7 illustrates respective relationships between the maximumenergy of electrons supplied from an electron supply system and thepercentage of breakdown caused in a gate oxide film, the thickness ofwhich is set at 3.5 and 5.0 nm, respectively. As can be understood fromFIG. 7, the higher the maximum energy of electrons, the higher thepercentage of breakdown caused in the gate oxide film. As also can beseen from FIG. 7, if the thickness of the gate oxide film is 5.0 nm,breakdown never fails to happen in the film when electrons having energyof 10 eV or more are introduced thereto. On the other hand, if thethickness of the gate oxide film is 3.5 nm, breakdown never fails tohappen in the film when electrons having energy of 7 eV or more areintroduced thereto.

[0074] Next, a relationship between the thickness of a gate oxide filmand the maximum energy of electrons at which breakdown happens in thegate oxide film will be described. As⁺ ions are herein implanted intothe gate oxide film of an MOS transistor, exhibiting an antenna effect,with the thickness of the gate oxide film varied, under the conditionsthat accelerating voltage is 20 keV, implant dose is 5×10¹⁵/cm² and beamcurrent is 10 mA. In the MOS transistor, p- and n-type substrates areused, a gate insulating film has an area of 1×10⁻⁶mm², the gateelectrode has an area of 1×10⁻¹mm² and the antenna ratio is 1×10⁵.

[0075]FIG. 8 illustrates a relationship between the thickness of a gateoxide film and the maximum energy of electrons at which breakdownhappens in the film. As can be understood from FIG. 8, the breakdownhappens in the gate oxide film when the maximum energy of electrons isapproximately 2 (eV/nm)×t (nm)=2t (eV). That is to say, electrons havingenergy of 2t (eV) cause a breakdown in the gate oxide film.

[0076] Based on these results, it can be understood that a negativecharge buildup phenomenon caused in a gate oxide film upon theintroduction of electrons depends on the maximum energy of the electronsintroduced into the film. Accordingly, if the maximum energy of theelectrons introduced is controlled at a predetermined value or less,then the charge buildup phenomenon in the gate oxide film can besuppressed.

[0077] Next, the relationship between the flux of electrons supplied andthe percentage of breakdown caused in a gate oxide film will beexamined. In the following description, the breakdown state of a gateoxide film will be analyzed while controlling the maximum energy ofelectrons supplied from an electron supply system at a predeterminedvalue or less and changing the flux of electrons supplied.

[0078] The relationship between the flux of electrons supplied and thepercentage of breakdown caused in a gate oxide film is analyzed underthe following conditions. As⁺ ions are herein implanted into the gateoxide film of an MOS transistor, exhibiting an antenna effect, with thethickness of the film varied, under the conditions that acceleratingvoltage is 20 keV, implant dose is 5×10⁻⁵/cm² and beam current is 10 mA.In the MOS transistor, p- and n-type substrates are used, a gateinsulating film has an area of 1×10⁻⁶ mm², the gate electrode has anarea of 1×10⁻¹ mm² and the antenna ratio is 1×10⁵.

[0079]FIG. 9 illustrates a relationship between the flux of electronssupplied and the percentage of breakdown caused in a gate oxide film. Onthe axis of abscissas, 0, 5, 10, 15, 20, 25 and 30 indicate respectivefluxes of electrons supplied and each numeral in parentheses indicatesan apparent amount of current on a wafer. That is to say, the numeralindicates a total of the flux of electrons supplied from an electronsupply system and the amount of beam current. In this case, if theamount of current is a positive value, then the flux of electrons is anegative value. Accordingly, the numeral indicates in actuality adifference obtained by subtracting the flux of electrons from the amountof beam current. As can be understood from FIG. 9, if the flux ofelectrons is equal to or smaller than 10 mA (i.e., if the apparentamount of current is equal to or larger than 0 mA), breakdown is verylikely to happen in the gate oxide film. On the other hand, if the fluxof electrons exceeds 10 mA (i.e., if the apparent amount of current issmaller than 0 mA), breakdown rarely happens in the gate oxide film. Andeven if the flux of electrons is as large as 30 mA, breakdown hardlyhappens in the gate oxide film.

[0080] These results tell that the breakdown in the gate oxide film hasnothing to do with the flux of electrons supplied, but is caused by acharge buildup phenomenon resulting from positive charges of the ionbeam.

[0081] As is apparent from the foregoing description, the followingconclusions can be drawn:

[0082] (1) The charge buildup phenomenon happening in a gate oxide filmdepends on the maximum energy of electrons introduced into the gateoxide film;

[0083] (2) The charge buildup phenomenon does not happen in the gateoxide film if the maximum energy of electrons is approximately 2t (eV)or less;

[0084] (3) The charge buildup phenomenon does not happen in the gateoxide film if only electrons of such a flux as to neutralize the beamcurrent are supplied; and

[0085] (4) As long as the maximum energy of electrons is too low tocause the charge buildup phenomenon in a gate oxide film, the phenomenondoes not happen in the film even though the flux of electrons hasincreased.

[0086] Thus, by adjusting arc current and bias voltage, supplied to theguide tube by an electron supply system, to control the maximum energyof electrons supplied at a value determined by the thickness of a gateoxide film or less, breakdown, happening in the gate oxide film due tothe negative charge buildup phenomenon, can be suppressed.

[0087] During an actual manufacturing process of devices, supposing thethickness of a gate oxide film is t (nm), it is possible to preventbreakdown from being caused in the gate oxide film due to the negativecharge buildup phenomenon so long as the maximum energy of electronssupplied is approximately 2 (eV/nm)×t (nm)=2t (eV) or less.

[0088] A conventional ion implanter does not have any function ofmeasuring the energy and flux of electrons supplied before, during andafter ion implantation. Thus, the energy of electrons during the ionimplantation cannot be known using such an apparatus. Also, since theenergy and flux of electrons supplied are unknown, the variation in fluxof electrons supplied resulting from the variation in state of the ionimplanter cannot be appropriately dealt with, which might also causebreakdown in a gate oxide film due to the charge buildup phenomenon.

[0089] In contrast, the apparatus of the invention includes: theelectron parameter sensor 111 including variable voltage supply 112,ammeter 113 and wafer voltage controller 114; the tube bias controller104 for controlling the level of the tube bias voltage supplied from thefirst voltage supply 103; and the arc current controller 110 forcontrolling the amount of the arc current supplied from the currentsource 109. Accordingly, the apparatus of the present invention cancontrol the maximum energy of electrons supplied from the electronsupply system 117 at a predetermined value or less, thus preventing thebreakdown in a gate oxide film due to the charge buildup phenomenon.

[0090] The respective operations of the electron parameter sensor 111,tube bias controller 104 and arc current controller 110 and the reasonwhy the breakdown, resulting in a gate oxide film from the negativecharge buildup phenomenon, can be prevented in this embodiment withoutfail, will be described.

[0091] First, before ions are implanted, the ion beam and electrons areintroduced into the wafer holder 100 under the same conditions as thosefor the ion implantation. In this case, as the voltage applied from thevariable voltage supply 112 is increased, the amount of current measuredby the ammeter 113 decreases, because electrons, having energy lowerthan the applied voltage, cannot reach the wafer holder 100.Accordingly, the amount of current is measured by the ammeter 113 withthe voltage applied from the variable voltage supply 112 increased atregular intervals, thereby calculating values of current associated withrespective applied voltages. And based on the values of currentassociated with the respective applied voltages, fluxes of electronswithin respective energy ranges are calculated. Also, in this case, anintegrated value of the fluxes of electrons within respective energyranges is calculated to derive the total flux of electrons supplied andthe maximum energy of electrons.

[0092] Next, suppose the maximum energy of electrons, supplied from theelectron supply system 117, is higher than the predetermined energy, atwhich the breakdown happens in the gate oxide film and which isdetermined by the thickness of the film in the device as a target of ionimplantation. Then, the wafer voltage controller 114 outputs controlsignals to the tube bias and arc current controllers 104, 110, therebyreducing the level of the tube bias voltage supplied from the firstvoltage supply 103 and the amount of the arc current supplied from thecurrent source 109.

[0093] By repeatedly performing this operation until the maximum energyof electrons supplied from the electron supply system 117 reaches thepredetermined energy (i.e., the maximum energy below which no breakdownhappens in the gate oxide film), the breakdown, resulting from thecharge buildup phenomenon in the film, can be prevented.

[0094] In this case, if the wafer voltage controller 114 can sense thethickness of the gate oxide film in the device as a target of ionimplantation, then the controller 114 can repeatedly output controlsignals to the tube bias and arc current controllers 104, 110 until themaximum energy of electrons is equal to or lower than the predeterminedenergy. Accordingly, the breakdown, resulting from the charge buildupphenomenon in the gate oxide film, can be automatically prevented.

[0095] In the foregoing embodiment, As⁺ ions are implanted under theconditions that accelerating voltage is 20 kev, implant dose is5×10¹⁵/CM² and beam current is 10 mA. The ion implantation conditions,however, are not limited to these exemplified ones, but may beappropriately modified in terms of ion species, accelerating voltage,implant dose and value of beam current.

[0096] Also, in this embodiment, an MOS transistor exhibiting an antennaeffect is used as an exemplary target device, in which the percentage ofbreakdown, resulting from a charge buildup phenomenon in the gate oxidefilm thereof, should be measured. Alternatively, any other transistormay be used so long as the transistor includes a gate electrode.

[0097] Moreover, in this embodiment, the energy and flux of electronssupplied by the electron supply system can be controlled using arccurrent and tube bias voltage as parameters. The present invention,however, is naturally applicable to a control over the energy and fluxof electrons supplied by the electron supply system using any parametersother than arc current and tube bias voltage.

[0098] Furthermore, any electron supply system other than theexemplified one, which supplies electrons, created from plasma, onto anorbit of an ion beam, may be used so long as the system can control theparameters used for controlling the energy of electrons.

What is claimed is:
 1. An apparatus for introducing an impurity, comprising: means for introducing an impurity having charges into a target to be processed, the target being a semiconductor substrate or a film formed on the substrate; means for supplying electrons into the target, the electrons canceling the charges of the impurity; and means for controlling the maximum energy of the electrons supplied by the electron supply means at a predetermined value or less.
 2. The apparatus of claim 1, wherein the impurity introducing means is means for implanting ions as the impurity.
 3. The apparatus of claim 1, wherein an insulating film with a thickness of t (nm) is formed on the semiconductor substrate, and wherein the predetermined value is 2t (eV).
 4. The apparatus of claim 1, further comprising means for measuring the energy of the electrons supplied by the electron supply means.
 5. The apparatus of claim 4, wherein the energy measuring means comprises means for measuring the maximum energy of the electrons supplied by the electron supply means.
 6. The apparatus of claim 4, wherein the energy measuring means makes the control means control the maximum energy of the electrons, supplied by the electron supply means, at the predetermined value or less based on the measured energy of the electrons.
 7. A method for introducing an impurity, comprising the steps of: introducing an impurity having charges into a target to be processed, the target being a semiconductor substrate or a film formed on a substrate; and supplying electrons from a filament contained in an arc chamber, to the target for neutralizing the charges of the impurity, wherein the step of supplying electrons includes the step of controlling a value of the maximum energy a value of the electrons supplied at a predetermined energy or less by changing an arc current flowing between the arc chamber and the filament, and a tube bias voltage applying between an earth and a guide tube that is arranged with an opening facing the target. 